Lightning arrester



Patented Sept. I, 1943 I UNITED STATES, PATENT OFFICE LIGHTNING ARRESTERRaymond R. my, Niagara Falls, N. Y., as-

signor to Norton Company, Worcester, Mass., a

corporation oi Massachusetts Application June 13, 1940, Serial No.340,237

17 Claims.

The invention relates to ,a lightning arrester and a composition ofmatter for use therein.

One object of the invention is to .provide Figure 2 is a diagram of atypical power line protected by arresters; and

Figure 3 is an oscillogram showing the performance of a typical arresterunder surge disgranular material for the manufacture of lightningarresters which prevent the establishment of high voltages on powerlines or other apparatus being protected. Another object of theinvention is to provide granular material for lightning arresters havinggood valving action. Another object of the invention is to providegranular material for a lightning arrester which will not readily core.Another object of the invention'is to provide a lightning arrester whichwill operate successfully over-a long period of time.

Anotherobject oi the invention is to provide a method of manufacture oflightning arrester material which is simple and readily controlled inpractical manufacture. Another object of the invention is to provide alightning arrester with high surge dischargecapacity without/physical orchemical breakdown of the material of the arrester. Another object ofthe invention is to provide lightning arrester grain of a compositionwhich can be duplicated with comparative ease and the elements of whichare readily available. Another object of the invention is to-providelightning arrester grain which will not clinker and which has anexcellent shut-off valve action.

Another object of the invention is to provide an arrester particularlyadapted to low voltage lines without providing unusual sizes and shapes.Another object of the invention is to provide a long arrester for highvoltages permitting the use of a thin-walled porcelain container ofsmall cross-section which is, therefore, inexpensive. Another object ofthe invention is to enhance the shut-01f characteristics of an arrester.

Other objects will be in part obvious or in part be pointed outhereinafter, and the scope of the application of which will be indicatedin the following claims.

In the accompanying drawing illustrating one of many possibleembodiments of the mechanical features of the invention,

Figure 1 is a vertical axial sectional view of an arrester constructedin accordance with the invention charge conditions.

Figure 4 is a curve showing resistance plotted against temperature forboron carbide and silicon carbide respectively, the resistance being ona log scale.

Referring now to Figure 2, I illustrate a typical power line, but thearrester may be usedin any circuit, that shown being illustrativemerely;

I H. The primary coil I3 is connected by leads [5 and I8 to the powerlines l0 and H, respectively. The secondary coil I4 is connected byleads I! and I8 to a low voltage secondary circuit and may operate, forexample, at 110 volts. To a midpoint on the secondary coil M isconnected a ground wire 20 grounded at 2|.

. I provide arresters and 26. Arrester 25 is connected by a line 21 tothe main power line I0 to protect the main power line ill from lightningdischarges and induced charges resulting from lightning bolts. Thelightning arrester 26 is connected by a line 28 to power line Hsimilarly to protect the power line H. The ar rester 25 is connected bya wire 30 to a groundwire 3|. The arrester 26 is connected by a wire 32to'the ground wire 3|. The ground wire 3| is also connected to theground wire 20 and to a ground 33. In the form of the invention hereinillustrated, the arresters 25 and 26 may be identical and may be of theloose grain type.

Referring to Figure 1, I provide a hollow cylindrical porcelain body 40having throughout the greater part of its length a unidiameter bore 4|and adjacent to one end thereof, an enlarged diameter bore 42. A metalplate 43 rests against an annular shoulder 44 joining the bores 4i and42 and forms with the bore 41 a chamber. The plate 43 is held againstthe shoulder 44 by means of a metal ring 45. A terminal 46 is attachedto the plate 43 and extends downwardly through base seals of plasticcement 41 and non-plastic cement 48.

At the other end of the lightning arrester is an arcing unit assemblyconsisting of a plurality of brass discharge plates 50 of saucer shapewith annular V peripheries. These are held together in spaced relationby means of a bottom plate 5| and bolt 52 attached thereto. A nut 53 andwashers 54 hold the parts together and the plates are spaced byporcelain insulators 55. A metal cap 56 fits the end of the porcelaincylinder II,

a portion of it being between the nut I3 and the washers 54. A terminal51 is suitably attached to the metal cap 56. A porcelain cap ll coversthe top of the arrester and keeps oisture and. dirt out of it.

I fill the space between the plates 43 and II with granular material 60according to the invention. This should be thoroughly compacted, and Iprovide an additional disk il having a semi-circular groove formedtherein by upsetting the metal, which receives a ring 52. -When the disk5i and the disk ii are pressed downwardly they and the ring 82 mayreadily move, but they resist any force tending to raise them by reasonof a wedging action.

Prior to this invention lightning arrester granules have been made ofsemi-conducting materials which are also refractory. Such materials havebeen primarily composed of silicon carbide of various degrees of purity.Prior to my invention the characteristic of refractoriness was alablevapor pressure at temperatures as high as 2300 C. There is no tendencyto melt until approximately 2500 C. is reached. It will be seen thatthis grain, therefore, provides a refractory conductor which exceeds thecapabilities of all previously used materials, because boron carbidedoes not vaporize at high temperatures as ddes silicon carbide.Furthermore boron carbide does not recrystallize as readily as siliconcarbide and thus there is less danger of coring. Therefore I have foundthat boron carbide constitutes a superior material for the valve oflightning arresters for certain practical installations.

One of the chief characteristics of boron carbide is its relatively highbody conductivity as compared with materials which have been customarilyused for lightning arrester valves. In accordance with the law W=I R amaterial of low resistivity can be embodied in a smaller valve than amaterial of high resistivity, the. heat evolved per unit volume of valvebeing the limiting factor. The importance of heat liberated per unitvolume has not heretofore beensufllciently appreciated, and I have foundthat under surge conditions the interfaces between grains areinstantaneously heated to temperatures sufficient to bring aboutchemical changes and the deterioration of the grains as valves.

A further advantage of using boron carbide is that since the grainitself has a high inherent conductivity within the body of the crystal,a much finer grit size can be used for equivalent conductance undersurge than can be used with a poorer conducting'material such as siliconcarbide. It is feasible to make arrester bodies of boron carbide withgrit size 150 mesh and liner and still have a low enough resistance topermit .the handling of large quantities of surge current withoutdamaging the arrester body because of power liberated within it. Theadvantage of using a larger number of contacts in a fixed volume of thearrester body resulting from the smaller grit size resides in theexcellent valvlng or shut-off characteristics. Thus boron carbide grainof small grit size will give excellent results for the type ofinstallations required with high voltages.

Referring now to Figure 4 there is therein shown the resistance of boroncarbide and silicon carbide respectively plotted against temperature,the resistance being given on a log scale. Cylinders 1.13 inches indiameter and, .125 inch long were packed with 90 grit size grains ofboron carbide and silicon carbide respectively. The resistance at thevarious temperatures is indicated in Figure 4. Since the slope of thecurve for boron carbide is steeper than that for silicon carbide it willbe seen that the boron carbide granules will increase in conductivitywith the current demands put upon them by the size of the surge morethan the silicon carbide; The outstanding difference between thematerials, however, is the much lower resistance of boron carbide.

Referring now to Figure 3, it is customary to use various electricaltests to examine the properties of silicon carbide which make it usefulfor a valve material in lightning arresters. Various manufacturers oflightning arresters have standardlzed on a wide variety of electricaltests to discover the suitability of a given lot of refractory V carbidematerial before such material is introduced into the arresters in loosegrain form or is manufactured into disks or blocks containing many typesof bonds. A desired electrical characteristic in the arrester is that ofdecreasing the apparent resistance to ground with increasing currentleakage while the current is rising in the arrester circuit. It is thisproperty which permits the arrester to discharge large surges to groundwithout having a peak voltage appear on the line which would destroy theinsulation of transformers and other electrical equipment which would beconnected to said line. It is transient peak voltages which do thedamage to the insulation of the line and the electrical equipmentattached thereto. Therefore, it is customary for the builders oflightning arresters to examine the transient phenomena by means of acathode-ray oscillograph where instantaneous voltage drops andcorresponding currents may be recorded for judging the performance ofthe valve material. A typical cathode-ray oscillogram of a lot of grainof my invention is herein given in Figure 3. The measurement was made onloose grain held in an insulating body under standard conditions ofpacking and with a fixed ratio of length to cross section. The pressureof the grain was standardized( The second and the fourth surges on theoscillogram correspond to the second and fourth artificial lightningstrokes applied to the arrester. In the case of this specific testplotted it will be noted that the size of the current stroke was morethan twice as great on the fourth as it was on the second test. Asuitable means for comparing the quality of arrester materials is tosubject them to surgespf increasing magnitude until they break down. Abreakdown has been shown in some unsuitable materials in 10microseconds. In such a case the voltage would go to zero and thearrester would act as a short circuit on the line. The voltage curveshown by the fourth surge is smooth and shows no breakdown. It alsoshows a very definite cut-off indicated by the sharp point of deflectionat 27 microseconds in the case of the second surge and at 25microseconds in the case of the fourth surge.

It is a feature of this invention that some of the advantages may beachieved by mixing a small amount of boron carbide grain with a largeramount of silicon carbide grain. The benefits ascribed to boron carbideand silicon carbide individually can be achieved in combination in thefollowing manner.

I provide silicon carbide of relatively large grit size. for example ofthe order of 60 mesh. I then coat this silicon carbide with boroncarbide of much finer size, for example 150 to 200 mesh. While there aremany practical ways of coating silicon carbide with boron carbide I mayattach the boron carbide to the silicon carbide with sodium silicate.For example if silicon carbide grains of the desired size are mixed witha sodium silicate solution containing one mol NazO to two mols of SiOzthen partially dried boron carbide can be mixed in with the grains andwill adhere to the silicon carbide. known to the art to form a mixedcarbide consisting in part of boron carbide and in part of siliconcarbide and a material of this nature may be used in carrying out myinvention. Boron carbide suitable for this application is preferablyfree from uncombined graphite, free boron, and suboxides of boron, thepresence of which destroys the desirable electrical properties which arecharacteristic of the material. Such a product is described in my UnitedStates Patent No. 1,897,214. The method or manufacture of this carbideis described in detail therein.

With regard to the proportions of boron carbide and silicon carbide,respectively, there is no 7"? limit since as aforesaid I can use pureboron carbideBrC. In order to have an appreciable advantage over plainsilicon carbide, it is sufficient to use one per cent of boron carbide.

It will thus be seen that there has been provided by this invention anarticlein which the various objects hereinabove set forth together withmany thoroughly practical advantages are successfully achieved. As manypossible embodiments may be made of the above invention and as manychanges might be made in the embodiment above set forth, it is to beunderstood that all matter hereinbeiore set forth, or shown in theaccompanying drawing, is to be interpreted as illustrative and not ina-limiting sense.

I claim:

1. A lightning-arrester valve-element comprising as its basic material,a loose mass of crystalline granules or the mixed carbide of boron andsilicon in which the proportion of silicon to the carbon of the mixedcarbide, not combined with boron is in the ratio S and the proportion ofboron to the remainder of the carbon in the mixed carbide is in theproportion B40, the mixed carbide having at least one per cent of boroncarbide.

2. A lightning-arrester valve-element comprising as its basic material,boron carbide, B40.

3. A lightning-arrester valve-element comprising as its basic material,silicon carbide 81C grains and boron carbide B40 grains, the boroncarbide grains being bonded to the silicon carbide grains and therebeing no less than one per cent of boron carbide.

4. A lightning-arrester valve-element oomph; ing as its basic material,silicon carbide 81C grains and boron carbide B40 grains, the boroncarbide grains being bonded to the; silicon carbide grains with alkalinesilicate and there being no less than one per cent of boron carbide.

5. A lightning-arrester valve-element in which the basic materialconsists of granular boron Furthermore it is now carbide 134C and noother substance in any substantial amount.

6. A lightning-arrester valve-element according to claim 3 in which theboron carbide grains ing to claim 4 in which the boron carbide grainsare substantially smaller than the silicon carbide grains.

8. A lightning arrester valve element comprising in combinationsilicon-carbide grains and means for lowering the resistance of saidsiliconcarbide grains consisting of boron-carbide grains bonded to saidsilicon-carbide grains.

9. A lightning arrester valve element comprising in combinationsilicon-carbide grains and means for lowering the resistance of saidsiliconcarbide grains consisting of boron-carbide grains bonded'to saidsilicon-carbide grains of substantially smaller grit size than saidsilicon-carbide grains intimately associated with said siliconcarbidegrains.

10. A lightning arrester valve element comprising in combination a massof silicon-carbide grains and means for lowering the resistance of themass of silicon-carbide grains comprising boron-carbide grainssubstantially smaller than said silicon-carbide grains, bonded to saidsiliconcarbide grains.

11. A lightning arrester valve element comprising in combination agranular mixture formed largely of silicon-carbide grains with which aremixed in intimate relation a relatively small amount of boron-carbidegrains.

12. A lightning arrester valve element comprising in combination agranular mixture formed largely of silicon-carbide grains with which aremixed in intimate relation a relatively small amount of boron-carbidegrains, the grit size of said boron-carbide grains being substantiallysmaller than the grit size of said silicon-carbide I grains.

13. A lightning arrester valve element compris ing silicon-carbidegrains coated with boroncarbide grains substantially smaller than saidsilicon-carbide grains.

14. A lightning arrester valve element comprising in combination arelatively large amount of silicon-carbide grains of large grit sizehaving means for lowering the resistance of said siliconcarbide grainswithout substantially increasing the space required for saidsilicon-carbide granules consisting of a relatively small amount ofboron-carbide grains of small grit size bonded to some of saidsilicon-carbide grains.

i5. Alightning arrester valve element compris ing a body of mixedgranules of silicon-carbide and boron-carbide'the volume of said bodybeing determined by the amount of silicon-carbide granules and saidboron-carbide granules being present in the interstices between saidsiliconcarbide granules 16. A lightning arrester valve elementcomprising silicon-carbide granules of a grit size on the order at 66mesh, having attached thereto boronv carbide granules of grit size onthe order of -200 mesh.

17. A lightning arrester valve element comprising silicon-carbidegranules of a grit size on the order of 60 mesh, compacted withboron-carbide granules of a grit size on the order of 150409 mesh.

RAYMOND R.

